For the people to clearly see an object, twp processes of locating the actual distance of the object to be seen and obtaining the clear image of the object formed on the retina are required in general. These two processes are generally so called convergence eye movement and eyeball adjustment. The eyeball adjustment is the process that the eyeball obtains the clear image of the object by changing the focus. The convergence eye movement is the process that the image of the object is formed right on the concave part of two eyes' macula fovea, i.e. the process that the two eyes locate the distance or depth of focus of the object. Generally, as two health eyes see the object, the convergence and the adjustment of the eyeballs are synchronous. Two eyes can locate the actual distance of object while the two eyes are obtaining the clear image of the object.
When people wears glasses and watches the images displayed by the 3D display screen, the location that the human eyes can obtain the clear image is the surface of the display screen, i.e. the position that the best clarity and resolution of the object can obtained are both the surface of the display screen. For the people to sense the 3D effect, the 3D display utilizes the binocular parallax theory. With a series of post productions to the images, people can sense that the objects are closer or farer to make the human eyes feel the depth of focus.
However, even though after the post productions, the convergence and the adjustment distances of the images shown by the 3D display are different. When people watch the 3D images and find out that the depths of focuses of the object and the currently seen object are different, the human eyes will rotate radially and inwards, i.e. the convergence eye movement. The depths of the focuses of the objects are different, and then, the ranges of the eyeball rotations are different, either. However, in the convergence process, the depth of the focus of the object can be confirmed but in the meantime, the clear image cannot be seen from the surface of the display screen cause of the eyeballs' rotations. Now, the eyeballs will get into the adjustment process for obtaining the clear images from the surface of the display screen. In a normal daily life, the distances of the convergence and the adjustment are the same. People often get dizzy and nausea due to the bad reactions brought by the visual fatigue after a long period of time of watching the 3D images because the conflict of the adjustment and the convergence shows due to the repeated adjustment and convergence. For easing the aforesaid conflict of the adjustment and the convergence, the range of depth of focus is controlled in the 3D image production to relieve the visual fatigue in general.
Nevertheless, such simple control to the range of depth of focus can merely relieve the visual fatigue to a certain extent without the significant improvement. Meanwhile, such fatigue can be light or serious depending on the people's physiques or other factors. Still, people suffer the bad reactions brought by the visual fatigue when they watch the 3D images after a long period of time.